The invention relates to a process for producing an oxygen sensing element capable of measuring the partial pressure of oxygen in sample gases. More particularly, the invention relates to a process for producing an oxygen sensing element suitable for use in an exhaust gas purifying system wherein the content of oxygen in the exhaust gas from the internal combustion engine of an automobile is measured, thereby to determine the content of unburnt hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas and, based on the measurement results, the air-fuel ratio is appropriately adjusted so that the efficiency of a catalyst for purifying the exhaust gas is enhanced.
An oxygen sensor is an oxygen concentration cell having a structure such that electrodes are mounted on the opposite sides of a solid electrolyte composed of a sintered ceramic material capable of conducting an oxygen ion. An electromotive force is produced across the solid electrolyte by the difference in the partial pressure between the oxygen in reference and sample gases contacting opposite sides of the solid electrolyte. The concentration of oxygen in the sample gas can be determined by measuring the electromotive force that is produced. As is well known, such an oxygen sensor is also employed, in combination with an electronic fuel injection device mounted on the internal combustion engine of an automobile or a platinum group catalyst layer provided upstream an exhaust gas flow from the internal combustion engine, to detect the concentration of oxygen in the exhaust gas being under an approximately equilibrium condition.
There has been a problem with conventional oxygen sensors mounted on automobiles, in that phosphorus, lead and the like contained in the exhaust gas penetrate into the oxygen sensing element and fill up the gaps between the solid electrolyte granules causing the lowering of the function of the oxygen sensing element. In order to solve such a problem, it is proposed that a protective layer of a refractory material having pores capable of passing the exhaust gas therethrough is provided on the outer surface of the oxygen sensing element (see, for example, Publicly Disclosed Japanese Patent Application (Kokai) No. 48-90294). The protective layer of this element performs a protective function for a certain period of use. However, when used for an extended period of time, the pores of the protective layer are filled up with phosphorus, lead and the like which lowers the function of the oxygen sensing element. Further, the protective layer may sometimes be peeled from the surface of the element upon use.
Such a protective layer of a refractory material is produced conventionally by dipping an oxygen sensing element provided with a protective layer thereon in a slurry of a metal oxide to coat the surface of the element with the slurry and, then, sintering the coated slurry. Alternatively, the protective layer may be produced by a plasma spray coating technique.